The use of a beam of radiant energy, for example, a beam of light from a pulsed, a Q-switched, or a continuous wave laser in the machining of a workpiece is known. Laser machining, such as drilling a hole on a workpiece, offers several advantages over more conventional machining techniques such as the use of a rotating drill bit, or other mechanical device. One outstanding advantage stems from the fact that the coherent radiant energy beam for a laser can be focused to an extremely fine point and, therefor, machining, more specifically, the drilling of a workpiece can be performed with a much higher degree of accuracy and resolution than is possible with the use of conventional techniques.
Some prior machining systems employ a laser. For instance, U.S. Pat. No. 3,700,850 entitled "Methods for Detecting the Amount of Material Removed by a Laser", and issued to R. M. Lumley, et al, discloses a numerically-controlled machining system, wherein the amount of material removed after each laser pulse is monitored by measuring the time taken for the shock waves generated by the laser pulse to traverse the remaining portion of the workpiece.
The principle and practice of solid state photoacoustic spectroscopy (PAS) for analyzing a given solid are also known to one skilled in that art. In PAS light energy is first absorbed by a solid, converted into an acoustic signal which is characteristic of that solid, and then converted into an electrical signal for analysis purpose. Typically, a laser is employed to provide a modulating light beam directed at a solid material to be analyzed. It is known that the solid material absorbs the modulating light beam in a way characteristic of the particular solid material. Any light absorbed by the solid material is converted, in part, or in whole into heat by non-radiative deexcitation processes within the solid. The primary source of the acoustic signal arises from the periodic time dependent heat flow from the solid to the surrounding gas. This time dependent periodic heat flow causes an oscillatory time dependent pressure in a small volume of gas at the solid-gas interface. An additional source of time dependent pressure in the gas can arise when the absorbing solid ablates and subsequently burns to release its heat of combustion in the form of heat. It is this motion of the gas which produces the acoustic signal that is characteristic of the solid (herein referred to as the photoacoustic characteristics of the solid). This acoustic signal is typically detected by a microphone and is subsequently processed. The solid material is then analyzed by examining its PAS signature. A more detailed analysis is beyond the scope here, and is, for instance, described in a book entitled "Optoacoustic Spectroscopy and Detection", edited by Y. H. Pao, Academic Press (1977).
Laser drilling has been employed to a variety of workpieces including printed circuit boards. A major concern in laser drilling of via hole sites in a multilayered printed circuit board is the lack of a real-time control system which permits the adjustment of the laser parameters to the specific conditions of the via hole site being drilled. As a result of the lack of a satisfactory real-time control system for laser drilling, abnormal operation of the laser may cause undetected drilling failures. Control of the drilling penetration is difficult to achieve since drilling by employing a laser is a non-tactile operation and there are no satisfactory methods to monitor the progress of the laser drilling process. The laser beam may be drilling through a composite layer each of which may be made of entirely different, or similar materials. Each composite layer of such a multilayered printed circuit board may have entirely different optical and absorption characteristics. In order to achieve a satisfactorily drilled hole through such a multilayered structure, different critical laser drilling parameters, such as pulse power, and pulse duration, may be required at each layer level of the composite printed circuit board. Furthermore, an undetected misregistration of the laser beam with the via hole site may lead to an improperly sited hole rendering the entire circuit board defective.